The majority of phosphoric acid is produced by the so-called wet process method, whereby phosphate rock is reacted or acidulated with sulfuric acid to produce phosphoric acid and calcium sulfate. The wet process methods are further classified by the type of calcium sulfate produced. The dihydrate processes produce calcium sulfate dihydrate (gypsum) and a phosphoric acid solution containing typically 22% to 32% P.sub.2 O.sub.5. The hemihydrate processes produce a stronger phosphoric acid solution (35-50% P.sub.2 O.sub.5) and produce calcium sulfate in the hemihydrate form. Typically, the dilute phosphoric acid solutions are concentrated by evaporation and processed into a variety of phosphate containing fertilizers.
Phosphate rocks contain a wide variety of impurities that either completely or partially dissolve in the phosphoric acid solution when the phosphate rock is acidulated. The phosphoric acid solutions are relatively high in impurities such as aluminum, iron, magnesium and furthermore may contain elevated levels of heavy metals such as vanadium, chromium, cadmium, nickel, etc. The presence of these impurities render the phosphoric acid unsuitable for certain uses, such as, for example, use in metal treatment, food ingredients or for use in the production of sodium tripolyphosphate and other potassium and sodium phosphates.
Present methods for purifying the wet process phosphoric acid solution, for example, through the use of solvent extraction processes or through chemical precipitation, add substantially to the expense of manufacturing a phosphoric acid solution sufficiently low in impurities to render it suitable for use in, for example, water treatment, food ingredients or the production of sodium and potassium phosphates.
The present invention is concerned with a low cost method for purifying phosphoric acid, particularly phosphoric acid produced by the so-called wet process method. It should be noted, however, that the herein described process may also be used for the purification of phosphoric acid produced by other methods.
It has been proposed that the metallic impurities can be removed from impure phosphoric acid by membrane filtration. Specifically, the metallic impurities can be removed as metal ion complexes (e.g., as sulfate and phosphate complexes) with nanofiltration membranes. Nanofiltration membranes have pore sizes of from about 0.001 microns to about 0.01 microns, and can comprise, for example, a molecular weight cutoff of about 200 atomic mass units (amu).
Nanofiltration membranes are known in the art, and are available with a number of different chemical constituencies. Chemical constituencies having particular application for the purification of phosphoric acid solutiods are those of the general class of compounds known as polyatnides.
Although polyamide-based membranes have been proposed for the purification of phosphoric acid solutions, such membranes have been found to suffer an unacceptably rapid performance degradation when utilized in such applications. Accordingly, it is desirable to develop new methods of phosphoric acid purification utilizing nanofiltration membranes which avoid such rapid degradation of polyamide-based membranes.
A prior art nanofiltration filter element 10 is described with reference to FIG. 1. Element 10 is a so-called "spiral wound" filter apparatus, in that the nanofiltration membrane sheets are wound around a center core (permeate collection tube) of the element. Element 10 comprises a perforated central core 12 and a plurality of flat membrane sheets 14 wound around central core 12. Membrane sheets 14 have low pressure sides and high pressure sides, with the high pressure side being the side exposed to a pressurized feed solution. The low pressure sides are separated by a permeate spacer material to allow the collection and transport of the filtered phosphoric acid solution (also called permeate). Membrane sheets 14 are separated by support material 16 (material 16 is a so-called "brine spacer"). An outer shell 18 encases membrane sheets 14 and support material 16. Membrane sheets 14 can comprise, for example, polyamide-based materials. Support materials 16 and perforated central tube 12 can comprise, for example, polysulfone-based materials.
In operation, a flow of crude phosphoric acid feed solution 20 enters into membrane sheets 14 and passes longitudinally through filter element 10. The feed solution is under sufficient hydraulic pressure to force a phosphoric acid filtrate to the low pressure side of membrane sheets 14. The filtrate then flows radially and longitudinally to the permeate collection tube 12.
Preferably, the flow through element 10 is at a pressure of from about 300 pounds per square inch gauge (psig) to about 2000 psig, and will be at a temperature of at least 110.degree. F. The high temperature reduces a viscosity of the acid to enable a reasonable flow of filtrate through element 10 and to reduce solution feedside pressure drop to minimize "telescoping" of the membrane sheets 14. The membrane elements are operated in a so-called cross-flow mode to minimize the build-up of rejected impurities at the membrane surfaces.
A flow rate through element 10 can be less than about 100 gallons per minute (gpm). As the phosphoric acid passes through element 10, it permeates membranes 14 and enters perforated central core 12 whereupon it is expelled as a purified phosphoric acid solution 22. The remainder of the feed phosphoric acid solution 20 is expelled as a non-purified solution 24. Non-purified solution 24 will typically comprise the vast majority of the contaminating metal ions of feed solution 20.